ARP variants: which mechanism lets a router answer an ARP request on behalf of a remote host so local senders can reach it via the router?

Introduction / Context:
Address Resolution Protocol (ARP) maps IP addresses to MAC addresses on a local link. In some designs, a router may need to respond to ARP queries for remote hosts to maintain reachability without readdressing or to bridge legacy segments. This is where Proxy ARP is used.

Given Data / Assumptions:

• Scenario: A local host ARPs for a destination that is not on the same subnet.
• We want a device to respond so traffic can be sent to a next hop transparently.
• Router capability: respond with its own MAC for a remote IP.

Concept / Approach:
Proxy ARP allows a router to “pretend” to be the remote host at Layer 2 by answering ARP with its own MAC. The sender then forwards frames to the router, which routes them onward. RARP performs the inverse mapping (MAC→IP) for diskless hosts, and Inverse ARP is used on NBMA networks (e.g., Frame Relay) to learn Layer-3 addresses from known DLCIs.

Step-by-Step Solution:

Verification / Alternative check:
On Cisco IOS, “ip proxy-arp” enables this behavior on an interface; packet captures show the router replying with its MAC for the remote IP.

Why Other Options Are Wrong:
Gateway DP: Not a standard ARP mechanism.
RARP: Client learns its IP from a server based on MAC—unrelated to proxying.
Inverse ARP: NBMA address discovery, not ARP proxy.

Common Pitfalls:
Using Proxy ARP as a substitute for correct subnetting; it can obscure topology and complicate troubleshooting. Prefer proper default gateways and routing when possible.

Final Answer:
Proxy ARP